Arrays and Pointers

Arrays and Pointers

As I mentioned earlier in this hour, pointers and arrays have a close relationship in C. In fact, you can make a pointer that refers to the first element of an array by simply assigning the array name to the pointer variable. If an array is referenced by a pointer, the elements in the array can be accessed with the help of the pointer.

For instance, the following statements declare a pointer and an array, and assign the address of the first element to the pointer variable:

char  *ptr_c;
char  list_c[10];
ptr_c = list_c;

Because the address of the first element in the array list_c is the beginning address of the array, the pointer variable ptr_c is actually now referencing the array via the beginning address.

Listing 12.3 demonstrates how to reference an array with a pointer.

TYPE
Listing 12.3. Referencing an array with a pointer.

1:  /* 12L03.c: Referencing an array with a pointer */
2:  #include <stdio.h>
3:
4:  main()
5:  {
6:     int *ptr_int;
7:     int list_int[10];
8:     int i;
9:
10:    for (i=0; i<10; i++)
11:       list_int[i] = i + 1;
12:    ptr_int = list_int;
13:    printf( "The start address of the array: 0x%p\n", ptr_int);
14:    printf( "The value of the first element: %d\n", *ptr_int);
15:    ptr_int = &list_int[0];
16:    printf( "The address of the first element: 0x%p\n", ptr_int);
17:    printf( "The value of the first element: %d\n", *ptr_int);
18:    return 0;
19: }

OUTPUT

After the executable 12L03.exe is run from a DOS prompt, the following output is printed on my screen:

ANALYSIS

C:\app>12L03
The start address of the array: 0x1802
The value of the first element: 1
The address of the first element: 0x1802
The value of the first element: 1
C:\app>

In Listing 12.3, an integer pointer variable, ptr_int, is declared in line 6. Then, an integer array, list_int, which is declared in line 7, is initialized by the list_int[i] = i + 1 expression in a for loop. (See lines 10 and 11.)

The statement in line 12 assigns the address of the first element in the array to the pointer variable ptr_int. To do so, the name of the array list_int is simply placed on the right side of the assignment operator (=) in line 12.

Line 13 displays the address assigned to the pointer variable ptr_int. The output shows that 0x1802 is the start address of the array. (You might get a different address on your machine.) The *ptr_int expression in line 14 returns the value referenced by the pointer. This value is the value contained by the first element of the array, which is the initial value, 1, given in the for loop. You can see that the output from the statement in line 14 shows the value correctly.

The statement in line 15 is equivalent to the one in line 12, which assigns the address of the first element to the pointer variable. Lines 16 and 17 then print out the address and the value kept by the first element, 0x1802 and 1, respectively.

In Chapter 16, "Applying Pointers," you'll learn to access an element of an array by incrementing or decrementing a pointer.